Metallic articles may be made with a directionally solidified grain structure to enhance their mechanical properties at elevated temperatures. In particular, it is possible to produce components of a complex design that may be subjected to high thermal and mechanical stresses, such as guide vanes or rotor blades of gas turbines, using directionally solidified casting.
In directional solidification, molten metal in a mold defining the shape of the article is cooled unidirectionally from one end of the mold. The metal solidifies first at the end from which heat is removed and then along the length of the mold as the temperature falls below the solidus temperature. The resulting structure has a number of grains that are elongated along the length of the mold parallel to the heat flow direction. The grain boundaries are parallel to the heat flow direction as well. The grains typically exhibit an oriented grain structure according to the fastest growing crystallographic direction or a seeded orientation introduced at the end first solidified. The grain orientation is selected to achieve good high temperature properties.
In service, the article made by directional solidification may be positioned such that the major mechanical loading is applied parallel to the heat flow direction during solidification. The orientation of the grain structure parallel to the heat flow direction places the greatest material strength in this direction. Additionally, the orientation of the grain boundaries parallel to the heat flow direction reduces the incidence of grain boundary creep. Directional solidification is used to fabricate cast articles of nickel-base superalloys to be used in the hottest portions of aircraft gas turbine engines.
Nickel-base super-alloys are attractive for turbine-engine applications because of their high-temperature strength and corrosion resistance. Particularly important for high temperature applications is the high-temperature creep resistance. To improve engine efficiency, it may be beneficial to increase the operating temperature range of turbine engines to higher temperatures. This has led to an evolution of nickel-base super-alloys, from polycrystalline alloys to directionally-solidified alloys to single-crystal alloys. By aligning the grain structure with the stress axis in directionally-solidified materials, substantial improvements in high-temperature creep resistance were realized.
When an article is directionally solidified, there may be casting defects, both of types common to all casting processes and also of types unique to directional solidification. These defects may often be manifested as cracks, particularly intergranular cracks, that extend parallel to the direction of the solidification. There may be other types of defects produced during solidification and also during service.
Directionally solidified articles are relatively expensive to produce. It is therefore beneficial to repair defects produced during casting or service, if such repair is feasible. Several methods exist for repairing worn or damaged directionally solidified articles. Repair methods include, for example, conventional fusion welding, plasma spray, and the use of a tape or slurry material containing a mixture of a binder and a metal alloy powder which is compatible with the substrate alloy.
In one approach, the defect may be repaired by a fusion welding process. Welding is also an attractive method for fabricating turbine engine components. However, welding is not used extensively because of the vulnerability of nickel-base super-alloys to hot cracking and strain age cracking. In addition, welding may critically alter the grain structure in these alloys, which may lead to poor performance. Nonetheless, welding is attractive because of its potential for large economic benefits. Furthermore, welding is potentially useful in component repair operations. However, when applied to directionally solidified articles, welding may result in an inadequate repair that has an inhomogeneous microstructure and whose mechanical properties are unacceptably low. The repaired article may also tend to be of less ductility than the defect-free article.
Accordingly, what is needed is a method of welding directionally solidified alloys that alleviates the problems of prior art welding of these alloys. Also what is needed is an method that is effective at repairing directionally solidified articles in a cost-effective and/or efficient manner.